Technology has advanced enough that two-legged robots handle practical tasks across most of human society. The world is at peace. Humanity itself has not changed. Into this stable world arrives Kazuki Tomonaga — a robot created by an entity called HIKARI for the purpose of Global Illumination, built with a precision that approaches the complexity of a human being. His assigned mission: research and understand human emotion. Through a series of encounters with different girls, what begins as programmed curiosity starts developing into something that resembles genuine feeling. Kazuki builds his own philosophy along the way — without yet realizing that the decisions ahead of him will matter far beyond anything he was designed for.